Up in Smoke: Marijuana Toasts Memory

If you can't remember the headline of this article or are already struggling to recall some of the words at the beginning of the story, try hard to recall how much pot you smoked in your youth.

A new study finds those who've used a lot of marijuana have worse memories and don't think as quickly.

It's not the first study to suggest pot hurts memory, but the findings are stark.

In one memory test, long-time uses remembered seven of 15 words, on average. Non-users remembered 12 of 15. On a decision-making test, those who had rarely smoked pot had impaired performance 8 percent of time, while long-term tokers had 70 percent impairment.

The results are detailed in the March 14 issue of the journal Neurology.

The study involved 64 people age 17 to 49 selected from a larger study group. They were split into three groups: those who had smoked four or more joints per week for more than 10 years; those who'd been smoking for five to 10 years; and those who had smoked at least once but not more than 20 times and not at all in the past two years.

The middle group consistently scored in between the other two.

"We found that the longer people used marijuana, the more deterioration they had in these cognitive abilities, especially in the ability to learn and remember new information," said Lambros Messinis of the Department of Neurology at the University Hospital of Patras in Patras, Greece.

A separate study in Neurology last year found higher blood flow velocity in the marijuana users even a month after they stopped smoking. Researchers said the change could help explain other studies that have revealed memory problems in pot smokers.

A Harvard Medical School study in 2003 found lasting memory impairment in people who had started smoking marijuana before age 17, when the brain is still forming.

And research published in November indicated that heavy marijuana use might put adolescents who are genetically predisposed to schizophrenia at greater risk of developing the brain disorder.

Some 3.1 million Americans age 12 and older use marijuana daily or almost daily, according to the National Institute on Drug Abuse. In 2004, 5.6 percent of 12th graders reported daily use of marijuana.

Robert Roy Britt

Rob was a writer and editor at Space.com starting in 1999. He served as managing editor of Live Science at its launch in 2004. He is now Chief Content Officer overseeing media properties for the sites’ parent company, Purch. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey, and in 1998 he was founder and editor of the science news website ExploreZone. He has a journalism degree from Humboldt State University in California.